While some tech fans have heard about Project Ara, they may not have heard about other modular gadgets — or even about the modular mobile revolution.

The kids who grew up playing with Transformers and Legos are now building mobile consumer electronics that work in the same way — you snap them together for the custom device you want.

The same trends that are driving mobile computing, wearable computing, the Internet of Self and the Internet of Things are also enabling the modular revolution: Powerful, low-power chips, Bluetooth LE, miniaturization, distributed manufacturing, 3D printing and of course Moore's Law, which never stops making electronics cheaper and faster.

The base model target for a basic Ara smartphone without modules (which Google calls an "endo") is $50, then each additional module is extra.

Users will be able to buy and add extra batteries, special cameras, more memory, an extra SIM card, special sensors, photo printers and more powerful antennas.

Ara phones will come with three Ara-specific apps: The Ara Configurer, which walks you through building your Ara phone; the Ara Manager, which enables you to to manage the modules; and the Ara Marketplace, which is the online store for buying new modules.

Google is rolling out the first wave of devices in Puerto Rico (in partnership with local carriers OpenMobile and Claro) because, according to the company, some three-quarters of the people who use the Internet there do so via smartphones.

When users buy their first Ara phones, they'll be able to choose from a menu of more than 20 module options, with hundreds expected eventually. Google also provides all the specs and information needed for companies to use 3D printers to make their own custom modules.

Some phones will be sold based on themes, such as photography or hiking.

Ara modules will be built and sold by Google, but also by any company that chooses to use Google's development kit. (One company alone, which is called YEZZ, has already created 56 Ara phone concept modules.)